LATE PROTEROZOIC GRANITOID MAGMATISM IN THE STATE OF SAO-PAULO, SOUTHEASTERN BRAZIL

The Precambrian basement in the state of Sao Paulo is made up of a series of metamorphic units exposed at various erosional levels and in part separated by large faults. A strong overprint of the Brasiliano cycle (700-500 Ma) is evident from isotopic data over most of the region; nevertheless, a large part of the metamorphic belts belong to older (Archaean to Middle Proterozoic) reworked sequences. About 30% of the total basement area consists of intrusive granitoids of Late Proterozoic age. These can be grouped into six major groups, on the basis of petrography, geochemistry, rock association, associated manifestations and (whenever possible) age relationships. Gneissic mangerite-charnockite-hastingsite granite association appear as folded tabular bodies within high-grade regions. The Sao Jose do Rio Pardo suite is chemically comparable to the "aluminous A-type" granitoid series, and its emplacement possibly signals a period of extensional tectonics prior to the onset of the Brasiliano cycle (approximately 1000 Ma?). A series of elongated batholiths (approximately 650 Ma) with predominant porphyritic hornblende-biotite granitoids is present mainly in high-grade areas. The compositions are generally granitic to granodioritic, but intermediate (e.g. monozodioritic) rocks are locally common. These high-K calc-alkaline suites are in some aspects comparable to the Cordilleran I-type granitoids, and their origin might be related to a subductional environment. Migmatitic granitoids constitute a peculiar association, occurring as a series of small bodies within high-grade allochthonous domains. These polyphase, heterogeneous, often nebulitic granitoids are source-related products of crustal anatexis. Syn- to late-orogenic Brasiliano granitoids in the low- to medium-grade domains are dominantly porphyritic calc-alkaline biotite granites and granodiorites, reminiscent of Caledonian I-types. In contrast, a peraluminous association (porphyritic biotite granitoid to equigranular two-mica granite) predominates in high-grade domains, which possibly reflects a different tectonic evolution for the two regions. Late- to post-orogenic Brasiliano granitoids intruded after the major orogenic development of the region (approximately 610-590 Ma). I-type calc-alkaline and aluminous A-type granitoids are geographically associated in the Itu belt, and appear to show some temporal and geochemical (?) overlapping. Wiborgitic facies and some Sn-W mineralization are locally present in the A-type suites. A post-orogenic extensional period is marked in the south of the area by the emplacement of shallow A-type granitoids in the Serra do Mar belt (< 590 Ma). This period was diachronous throughout the region, as shown by the rather young (590-540 Ma) deformation ages registered in the Ubatuba Domain.